This invention relates to a tubular reactor, and for processes of conducting liquid/liquid multiphase reactions, particularly nitrations of aromatic compounds, in tubular reactors.
Reactions between immiscible or only slightly miscible liquids are commonly performed. Typical such reactions include an aqueous phase that is reacted with an immiscible organic phase. Because the desired is chemical reactions usually occur mainly at the interface of the liquid phases, an important factor in obtaining a complete reaction or a commercially acceptable rate of reaction is to intensely mix the phases. There are various ways of accomplishing this. A common way is to conduct the reaction with mechanical mixing, such as by using an agitator blade. Cascade reactors are also known. Apparatus of these types have various shortcomings. Moving parts tend to wear out and need maintenance or replacement. Usually the apparatus is relatively expensive. Often, back-mixing occurs, leading to the formation of undesired byproducts or in some cases, over-reaction of the raw materials.
The problems associated with reactions between immiscible liquids are illustrated well by the nitration of aromatic compounds. Two commercially important nitrated aromatic compounds are mononitrobenzene (MNB) and dinitrotoluene, which are prepared by nitrating benzene and toluene, respectively. MNB is a common solvent and can be converted to another commercially important compound, aniline. Similarly, nitrated toluene products such as dinitrotoluene are used to make derivatives such as toluene diamine, which can be further converted to toluene diisocyanate, an important raw material for making polyurethane polymers.
Aromatic ring nitration reactions are ordinarily conducted by mixing the aromatic compound with nitric acid in the presence of sulfuric acid. An adiabatic process for producing mononitrobenzene is described in U.S. Pat. No. 2,256,999 to Castner. In Castner""s process, as in all similar benzene nitration processes, the acids form a phase that is immiscible with the aromatic compound. Consequently, Castner describes using a series of agitated tanks for conducting the reaction in order to obtain a commercially acceptable reaction rate. However, the Castner process suffers from several difficulties, primarily low yields and the formation of high levels of nitrophenolic impurities. In addition, the Castner process forms undesirably high levels of overnitrated products, primarily dinitrobenzene.
The reliance on high shear mixing to deal with immiscible raw materials is reflected in other nitration processes as well. In U.S. Pat. Nos. 4,021,498 and 4,091,042, Alexanderson et al. describe using a xe2x80x9cvigorously agitatedxe2x80x9d tubular reactor to conduct the reaction. This alone was not sufficient to satisfactorily produce the desired product, however. Consequently, Alexanderson et al. require careful selection of the proportions of starting materials in order to reduce the level of impurities in the product. This general approach to reducing impurities was continued in U.S. Pat. No. 5,313,009 to Guenkel et al., in which impurity formation is said to be reduced using a specially designed mixer, which produces extremely fine benzene bubbles in the acid phase, followed by a tubular reactor that may include additional static mixing elements. Like Alexanderson et al., Guenkel et al. found that very specific proportions of starting materials were necessary in order to obtain a product with low levels of impurities.
Other references are similar. In U.S. Pat. No. 3,431,312 to Engelbert et al., nitration is performed in a series of cascade reactors, all of which are equipped with mixers or stirrers. In U.S. Pat. No. 4,973,770 to Evans, the reaction is performed by forming a turbulent jet of nitric and sulfuric acid to produce droplets of mixed acid having a size of from less than 1 xcexcm to about 10 xcexcm in diameter and contacting the acid droplets with the nitratable organic compound. In U.S. Pat. No. 5,963,878, a pipe nitrator discharges into a stirred tank type reactor.
In the process described in U.S. Pat. No. 5,763,687 to Morisaki, the reaction is conducted in a tube or pipe reactor equipped with a number of specially designed static mixing elements.
Thus, nitrations of aromatic compounds typify many of the problems that attend multiphase liquid/liquid reactions. On the one hand, for economic reasons it is necessary to obtain an acceptable reaction rate, and this is usually facilitated by increasing the contact between the phases. On the other hand, over-contacting the phases can cause impurities, particularly nitrophenols and cresols to form. Similarly, back-mixing or over-contacting the phases may cause even the desired reaction to go too far. With nitration reactions, this is seen in the production of over-nitrated products such as dinitrobenzene (in MNB production). The formation of impurities in this manner reduces yield, thereby reducing the overall economic efficiency of the process.
Thus, it would be desirable to provide an apparatus with which multiphase liquid/liquid reactions can be conducted, which provides good control of the reaction and efficient mixing of the phases. It would also be desirable to provide a process for conducting multiphase liquid/liquid reactions efficiently, with good yields and low levels of impurities and byproducts being formed. In particular, it would be desirable to provide a method of nitrating aromatic compounds with good yields, low levels of nitrophenolic impurities and low levels of undesired by-products, using relatively inexpensive equipment.
In one aspect, this invention is a tubular reactor comprising a tube having an inlet end into which a reaction mixture enters the tubular reactor, an outlet end from which a product stream emerges, and, located in said tube between said inlet and outlet ends, a sequence of short static mixing elements separated by coalescing zones, wherein (a) the length of each static mixing element is no greater than about 6 times the diameter of that static mixing element, and (b) the length of each of said coalescing zones is at least about 4 times the diameter of that coalescing zone.
The tubular reactor of the first aspect provides a simple apparatus in which multiphase liquid/liquid reactions can be conducted with good yields and low levels of impurities and by-products.
In a second aspect, this invention is a process for conducting a multiphase liquid/liquid reaction, comprising (1) introducing a stream of a multiphase liquid reaction mixture into an inlet end of a tubular reactor having a sequence of short static mixing elements separated by coalescing zones, wherein (a) the length of each coalescing zone is selected together with a flow rate of the reaction mixture such that as said reaction mixture passes through said coalescing zone, droplets of at least one liquid phase of the multiphase reaction mixture coalesce and at least partially phase separate from at least one other liquid phase of said reaction mixture, forming a topmost portion of said reaction mixture rich in one phase and a bottommost portion of said reaction mixture poor in said one phase, and when said reaction mixture passes from a coalescing zone through a static mixer element, said topmost and bottommost portions of said reaction mixture are sheared and blended to redisperse the coalesced droplets as smaller droplets in said at least one other liquid phase, (2) passing said reaction mixture under reaction conditions through said tubular reactor, and (3) withdrawing a stream containing a desired reaction product from an outlet end of the tubular reactor.
In a third aspect, this invention is a process for nitrating an aromatic compound, comprising passing under reaction conditions a reaction mixture including an aromatic compound and an acid phase containing sulfuric or phosphoric acid, nitric acid and water through a tubular reactor having a sequence of short static mixing elements separated by coalescing zones, wherein (a) the length of each of said coalescing zones is selected together with a flow rate of the reaction mixture such that as said reaction mixture passes through a coalescing zone, droplets of the aromatic compound coalesce and at least partially phase separate from said acid phase, forming a topmost portion of said reaction mixture rich in the organic compound and a bottommost portion of said reaction mixture poor in the organic compound, and when said reaction mixture passes from a coalescing zone through a static mixer element, said topmost and bottommost portions are sheared and blended to redisperse the aromatic compound as small droplets in the acid phase.
This process gives excellent yields of the desired nitration product, with low levels of under- and overnitrated products and nitrophenol impurities. In addition, low pressure drops in the tubular reactor allow the use of smaller pumping equipment, thereby reducing capital costs and energy consumption.